Residual or additive phase noise is a measure of noise added to an input signal by a two-port device under test (DUT), such as an amplifier, mixer, frequency converter, multiplier, and the like. Residual phase noise measurements start from the same basic principle. A component is driven by a sinusoidal signal source, and then the noise contributed by the signal source is cancelled out, leaving the noise contributed by the DUT.
FIG. 1 is a simplified block diagrams showing a conventional test setup for measuring residual phase noise.
More particularly, FIG. 1 depicts conventional system 100 signal, including signal source 110 for generating a stimulus signal, which is divided into first and second signals by splitter 115. The first and second signals are phase coherent since they are provided by the same signal source 110. The first signal is provided to DUT 105 on a first path, which includes first attenuator 121 and second attenuator 122. The second signal is provided to a second path, which includes mechanical phase shifter 130 for shifting the phase of the second signal approximately 180 degrees in relation to the first signal. The first and second signals are added at mixer 140, which physically cancels the carrier, leaving residual phase noise introduced by the DUT 105. For example, the first and second signals are supplied as the radio frequency (RF) and the local oscillator (LO) of the mixer 140, resulting in an intermediate frequency (IF) signal output at DC as close to zero as possible.
The IF signal is amplified by low noise amplifier (LNA) 150 and converted to digital data by analog-to-digital converter (ADC) 160. The DC centered spectrum of the IF signal may then be measured using a low frequency spectrum analyzer (not shown) to identify spurious signals (e.g., residual phase noise) above a predetermined level with a specified span of the DC IF.
Conventional residual phase noise measurement techniques have various drawbacks. For example, the phase shifter 130 would typically not be broadband. It is therefore difficult to design test systems capable of measuring the residual phase noise on broadband devices, such as a broadband amplifiers and converters. Also, there are fewer options for programmable phase shifters, further complicating design of an automated system.
In addition, conventional residual phase noise measurement techniques require that the mixer 140 (i.e., the phase detector) be a double balanced mixer. That is, the first and second paths provide signals driving the LO and RF ports of the mixer 140, respectively, and baseband analysis is made at the DC coupled IF port of the mixer 140. Although the mixer 140 thus may be used as the phase detector, this is typically not an application specified by manufacturers, resulting in significant trial and error efforts in locating appropriate parts. Also, diode selection, port isolation and IF circuit topology of the mixer 140 all affect performance. It is best to drive the mixer 140 under recommended power conditions, which typically means driving the LO port at approximately 13 dBm (driving an LO from 10 to 16 dBm is typical for a mixer specified at 13 dBm), and driving the RF port of the mixer 140 at approximately 5 dB lower than the LO port. A higher power level at the RF port provides more sensitivity in measurements, but only up to a certain point. At higher RF drive levels, the mixer 140 may add additional shot noise, ultimately masking the residual phase noise measurement.
Conventional residual phase noise measurement techniques further require an additional calibration step to relate the measured Vrms from the phase detector (e.g., mixer 140) to the added noise in dBm/Hz. Also, two different detectors, a voltmeter to determine quadrature and a baseband FFT based detector are needed for conducting measurements. Accordingly, more efficient means are needed for determining residual phase noise.